Optical multiplexing device

ABSTRACT

An optical multiplexing device spatially disburses collimated light from a fiber optic waveguide into individual wavelength bands, or multiplexes such individual wavelength bands to a common fiber optic waveguide or other destination. The optical multiplexing device has application for dense channel wavelength division multiplexing (WDM) systems for fiber optic telecommunications, as well as compact optical instrument design. Multiple wavelength light traveling in a fiber optic waveguide is separated into multiple narrow spectral bands directed to individual fiber optic carriers or detectors. An optical block has an optical port for passing the aforesaid multiple wavelength collimated light, and multiple ports arrayed in spaced relation to each other along a multiport surface of the optical block. A continuous, variable thickness, multi-cavity interference filter extends on the multiport surface of the optical block over the aforesaid multiple ports. At each of the multiple ports the continuous interference filter transmits a different wavelength sub-range of the multiple wavelength collimated light passed by the optical port, and reflects other wavelengths. Multicolor light passed to the optical block from the optical port is directed to a first one of the multiple ports on an opposite surface of the optical block. The wavelength sub-range which is &#34;in-band&#34; of such first one of the multiple ports is transmitted through that port by the local portion of the continuous, variable thickness interference filter there, and all other wavelengths are reflected. The light not transmitted through the first port is reflected to strike a second port, at which a second (different) wavelength band is transmitted and all other light again reflected. The reflected optical signals thus cascades in a &#34;multiple-bounce&#34; sequence down the optical block of the multiplexing device, sequentially removing each channel of the multiplexed signal. In reverse operation, individual channels are combined in the optical block and transmitted through the optical port.

This is a continuation of application Ser. No. 08/490,829, filed Jun.15, 1995,now U.S. Pat. No. 5,583,683.

INTRODUCTION

The present invention is directed to an optical multiplexing devicewhich spatially disburses collimated multi-wavelength light from a fiberoptic waveguide into individual wavelength bands, each of which can bedirected to an individual fiber optic waveguide output line, lightdetector, etc., or multiplexes such individual wavelength bands to acommon fiber optic waveguide or other destination. In certain preferredembodiments, the improved multiplexing devices of the present inventionare particularly well suited for dense channel wavelength divisionmultiplexing (DWDM) systems for fiber optic telecommunications systems.

BACKGROUND

While fiber optic cable is finding widespread use for data transmissionand other telecommunication applications, the cost of new installedfiber optic cable presents a barrier to increased carrying capacity.Wavelength division multiplexing (WDM) would allow different wavelengthsto be carried over a common fiber optic waveguide. Presently preferredwavelength bands for fiber optic transmission media include thosecentered at 1.3μ and 1.55μ. The latter is especially preferred becauseof its minimal absorption and the commercial availability of erbiumdoped fiber amplifiers. It has a useful band width of approximately 10to 40 nm, depending on application. Wavelength division multiplexing canseparate this band width into multiple channels. Ideally, the 1.55μwavelength band, for example, would be divided into multiple discreetchannels, such as 8, 16 or even as many as 32 channels, through atechnique referred to as dense channel wavelength division multiplexing(DWDM), as a low cost method of substantially increasing long-haultelecommunication capacity over existing fiber optic transmission lines.Wavelength division multiplexing may be used to supply video-on-demandand other existing or planned multimedia, interactive services.Techniques and devices are required, however, for multiplexing thedifferent discreet carrier wavelengths. That is, the individual opticsignals must be combined onto a common fiber optic waveguide and thenlater separated again into the individual signals or channels at theopposite end of the fiber optic cable. Thus, the ability to effectivelycombine and then separate individual wavelengths (or wavelength bands)from a broad spectral source is of growing importance to the fiber optictelecommunications field and other fields employing optical instruments.

Optical multiplexers are known for use in spectroscopic analysisequipment and for the combination or separation of optical signals inwavelength division multiplexed fiber optic telecommunications systems.Known devices for this purpose have employed, for example, diffractiongratings, prisms and various types of fixed or tunable filters. Gratingsand prisms typically require complicated and bulky alignment systems andhave been found to provide poor efficiency and poor stability underchanging ambient conditions. Fixed wavelength filters, such asinterference coatings, can be made substantially more stable, buttransmit only a single wavelength or wavelength band. In this regard,highly improved interference coatings of metal oxide materials, such asniobia and silica, can be produced by commercially known plasmadeposition techniques, such as ion assisted electron beam evaporation,ion beam spattering, reactive magnetron sputtering, e.g., as disclosedin U.S. Pat. No. 4,851,095 to Scobey et al. Such coating methods canproduce interference cavity filters formed of stacked dielectric opticalcoatings which are advantageously dense and stable, with low filmscatter and low absorption, as well as low sensitivity to temperaturechanges and ambient humidity. The theoretical spectral performance of astable, three-cavity filter (tilted 12°) produced using any of suchadvanced, deposition methods is shown in FIG. 1 of the appendeddrawings. The spectral profile is seen to be suitable to meet stringentapplication specifications.

To overcome the aforesaid deficiency of such interference filters, thatis, that they transmit only a single wavelength or range of wavelengths,it has been suggested to gang or join together multiple filter units toa common parallelogram prism or other common substrate. Optical filtersare joined together, for example, in the multiplexing device of U.K.patent application GB 2,014,752A to separate light of differentwavelengths transmitted down a common optical waveguide. At least twotransmission filters, each of which transmits light of a differentpredetermined wavelength and reflects light of other wavelengths, areattached adjacent each other to a transparent dielectric substrate. Theoptical filters are arranged so that an optical beam is partiallytransmitted and partially reflected by each optical filter in turn,producing a zigzag light path. Light of a particular wavelength issubtracted or added at each filter (depending upon whether the elementis being used as a multiplexer or demultiplexer). Similarly, in thedevice of European patent application No. 85102054.5 by Oki ElectricIndustry Co., Ltd., a so-called hybrid optical wavelength divisionmultiplexer-demultiplexer is suggested, wherein multiple separateinterference filters of different transmittivities are applied to theside surfaces of a glass block. A somewhat related approach is suggestedin U.S. Pat. No. 5,005,935 to Kunikani et al, wherein a wavelengthdivision multiplexing optical transmission system for use inbi-directional optical fiber communications between a central telephoneexchange and a subscriber employs multiple separate filter elementsapplied to various surfaces of a parallelogram prism. Alternativeapproaches for tapping selective wavelengths from a main trunk linecarrying optical signals on a plurality of wavelength bands issuggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. In thatpatent, multiple filter taps, each employing dielectric mirrors andlenses for directing optical signals, are shown in an arrangement forremoving a series of wavelength bands or channels from a main trunkline.

Applying multiple separate filter elements to the surface of a prism orother optical substrate involves significant disadvantages in assemblycost and complexity. In addition, a significant problem associated withwavelength division multiplexing devices and the like employing multiplediscreet interference filter elements, arises from uncertainty as to theprecise wavelength of a filter element as it is manufactured. That is,in the manufacture of multiplexing devices, wherein bandpass filterelements are produced separately, a device employing eight individualbandpass filters, for example, typically will require considerably morethan eight coating lots to produce the necessary eight suitable filterelements.

Bandpass filters (particularly in the infrared range) are extremelythick and require complicated and expensive vacuum deposition equipmentand techniques. Accordingly, each coating lot can be expensive anddifficult to produce. For this reason, devices employing, for example,eight separate interference filter elements to produce an eight channelWDM device, have been relatively costly and have not enjoyed fullcommercial acceptance.

Another problem associated with optical multiplexing devices employingmultiple individual bandpass filter elements, involves the need to mountthe elements in nearly perfect parallelism on an optical substrate. Thefilter elements are quite small, typically being on the order of 1 to 5mm in diameter, and are, accordingly, difficult to handle withprecision. Improper mounting of the filter elements can significantlydecrease the optical accuracy and thermal stability of the device. Arelated problem is the necessity of an adhesive medium between thefilter element and the surface of the optical substrate. The opticalsignal path travels through the adhesive, with consequent systemdegradation. In optical multiplexing devices intended for thetelecommunications industry, preferably there is as little as possibleepoxy adhesive in the optical signal path.

It is an object of the present invention to provide improved opticalmultiplexing devices which reduce or wholly overcome some or all of theaforesaid difficulties inherent in prior known devices. Particularobjects and advantages of the invention will be apparent to thoseskilled in the art, that is, those who are knowledgeable and experiencedin this field of technology, in view of the following disclosure of theinvention and detailed description of certain preferred embodiments.

SUMMARY OF THE INVENTION

In accordance with a first aspect, an optical multiplexing devicecomprises an optical block which may be either a solid opticalsubstrate, such as glass or fused silica or the like, or an enclosedchamber which is hollow, meaning either evacuated or filled with air orother optically transparent medium. The optical block has an opticalport for passing multiple wavelength collimated light. Depending uponthe application of the optical multiplexing device, such multiplewavelength collimated light may be passed through the optical port intothe optical block to be demultiplexed, or from the optical block as amultiplexed signal to a fiber optic transmission line or otherdestination. Multiple ports are arrayed in spaced relation to each otheralong a multiport surface of the optical block. As illustrated below inconnection with certain preferred embodiments, the optical block mayhave more than one such multiport surface. Each of these multiple portsis transparent to the optical signal of one channel. Thus, eachtransmits a wavelength sub-range of the multiple wavelength collimatedlight passed by the optical port. In an application of the opticalmultiplexing device in a multi-channel telecommunication system, each ofthe multiple ports typically would pass a single discreet channel and,in combination, the channels form the aforesaid multiple wavelengthcollimated light transmitted by the optical port. A continuous, variablethickness interference filter, preferably a multi-cavity interferencefilter, is carried on the multiport surface of the optical block toprovide the aforesaid multiple ports. Because this continuousinterference filter extending over the multiport surface has a differentoptical thickness at each of the multiple ports, the wavelength (orwavelength range) passed by the filter at each such port will differfrom that passed at the other ports. Thus, a single film, preferablydeposited directly onto the surface of the optical block, separatelypasses optical signals for each of a number of channels at separatelocations, while reflecting other wavelengths. As noted above, theoptical block may be either solid or a hollow chamber. In the case of asolid optical block, the multiport surface carrying the continuous,variable thickness interference filter would typically be an exteriorsurface of the block. As discussed further below, the individual portsof the multiport surface may be bandpass filters, preferably narrowbandpass filters transparent to a wavelength sub-range separated fromthe sub-range of the next adjacent port(s) by as little as 2 nm or evenless for DWDM. Alternatively, some or all of the multiple ports could bedichroic, i.e., a long wavepass or short wavepass edge filter,preferably with a very sharp transition point. The transition point ofeach port would be set at a slightly (e.g., 2 nm) longer (or shorter)boundary wavelength. In a demultiplexing operation, each port, in turn,would pass or transmit only optic signals in the incremental rangebeyond the boundary wavelength of the previous port, since all light atshorter (or longer) wavelengths would already have been removed. Lightbeyond the boundary wavelength of the new port would be reflected, inaccordance with the above described principles of operation.

The optical multiplexing device further comprises means for cascadinglight within the optical block along a multi-point travel path from oneto another of the multiple ports. In a demultiplexing operation, theoptical signals would enter the optical block at the aforesaid opticalport and travel to the multiple ports (acting in this case as outputports) along the aforesaid multi-point travel path. The signal for eachindividual channel is transmitted out of the optical block at itscorresponding port; other wavelength are reflected, or bounced, back tocascade further along the optical travel path within the optical block.It will be understood that at the last output port(s) there may be noremainder light to be reflected. It will also be understood from thisdisclosure, that the optical multiplexing device can operate in thereverse or both directions. The cascading means preferably comprises areflective film carried on a second surface of the optical block, eitheras a continuous coating spanning the multi-point travel path of thecascading light signals, or as multiple discreet reflector elements. Theoptical block would most typically be rectilinear, having the reflectivefilm on or at a second surface of the optical block opposite andparallel to the multiport surface carrying the aforesaid continuousinterference filter. This second film can be a broadband high reflector,that is, a film coating which is highly reflective of all wavelengths ofthe channels which combine to form the multiple wavelength collimatedlight, or can act as a second interference filter transparent at spacedlocations (i.e., at some or each of the bounce points) to the opticalsignal of one or more of the channels. In either case, the interferencefilter and reflective film on spaced surfaces of the optical blockoperate to cascade optical signals through the optical block in amultiple-bounce sequence starting at (or finishing at) the optical portthrough which the multiple wavelength collimated light passes. Thismultiple-bounce cascading will be further described below in connectionwith certain preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention are discussed below withreference to the accompanying drawings in which:

FIG. 1 is a graph showing the theoretical performance of a high qualitymulti-cavity, dielectric, optical interference filter.

FIG. 2 is a schematic illustration of a first preferred embodiment of anoptical multiplexing device, specifically, a dense channel wavelengthdivision multiplexing device for an eight channel fiber optictelecommunications system or like application;

FIG. 3 is a schematic illustration of an alternative preferredembodiment of an optical multiplexing device in accordance with theinvention, specifically, a dense channel wavelength divisionmultiplexing device for an eight channel fiber optic telecommunicationssystem or like application;

FIG. 4 is a schematic illustration of another alternative preferredembodiment of an optical multiplexing device in accordance with theinvention, specifically, a dense channel wavelength divisionmultiplexing device for an eight channel fiber optic telecommunicationssystem or like application; and

FIGS. 5 and 6 are schematic illustrations, in cross-section, of thecontinuous, variable thickness, three cavity interference filter of theoptical multiplexing device of FIG. 2.

It should be understood that the optical multiplexing devices andinterference filter illustrated in the drawings are not necessarily toscale, either in their various dimensions or angular relationships. Itwill be well within the ability of those skilled in the art to selectsuitable dimensions and angular relationships for such devices in viewof the foregoing disclosure and the following detailed description ofpreferred embodiments.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The optical multiplexing device, as disclosed above, has numerousapplications including, for example, in fiber optic telecommunicationsystems. Optical multiplexing devices of this design are particularlyuseful, for example, in test equipment and the like, as well aslaboratory instrumentation. For purposes of illustration, the preferredembodiments described below in detail are dense channel wavelengthdivision multiplexing devices which can solve or reduce the abovedescribed problems of individually mounting multiple filter elements toan optical substrate for each individual signal channel, the problems ofcost and complexity involved in multiple coating lots for preparing suchindividual filter elements, and the associated problems of filterwavelength uncertainty.

As discussed below in connection with the appended drawings, a gradedwavelength all-dielectric narrow bandpass filter is placed on at leastone side of an optical block, preferably a polished parallel plate ofspecific thickness. The filter, forming a continuous coating over atleast a portion of the surface of the optical block, preferably is amulti-cavity, most preferably three cavity, film stack coating modeledafter a Fabry-Perot etalon, and may be referred to simply as a cavityfilter. Two dielectric thin film stacks which by themselves form areflector for the optical wavelengths in question, are separated by athicker cavity layer. This structure is then repeated one or more timesto produce a filter with enhanced blocking and improved in-bandtransmission flatness. The net effect is to produce a narrowbandtransmissive filter where in-band light is transmitted and out-of-bandlight is reflected. As noted above, dichroic filters may also be used.This improved filter performance provides commercially acceptable densechannel wavelength division multiplexing for fiber optictelecommunications applications of the optical multiplexing device. Itprovides low cross-talk between channels and permits a suitably highnumber of channels within a given band width. An excessive number ofcavities will adversely affect the transmissivity of even in-bandwavelengths and may increase production costs for the opticalmultiplexing device beyond commercially acceptable levels.

The continuous, variable thickness, multi-cavity interference filter canbe produced with dense, stable metal oxide film stackes using thedeposition techniques mentioned above. Such filters have beendemonstrated to have excellent thermal stability, e.g., 0.004 nm/° C. at1550 nm, and ultra-narrow band widths, separated by as little as 2 nm,or even as little as 1 nm. Suitable variable thickness filters have beenused in other applications such as, for example, in U.S. Pat. No.4,957,371 to Pellicori et al. Stable Ultra-Narrowband Filters also areshown in SPIE Proceedings 7/1994. Preferably, the interference filter iscontinuously linearly variable in thickness. Optionally, however, thethickness of the continuous filter may be variable non-continuously f orexample, having a substantially uniform thickness over each of themultiple ports of the optical block associated with the separatechannels of the fiber optic system.

The interference filter typically comprises two materials, one of a highrefractive index such as niobium pentoxide, titanium dioxide, tantalumpentoxide and/or mixtures thereof e.g., niobia and titania, etc. At 1.5microns wavelength, the refractive index value for these materials isroughly 2.1 to 2.3. The low refractive index material is typicallysilica, having a refractive index of about 1.43. An interference filterhas an "optical thickness" which is the numerical product of itsphysical thickness times its refractive index. The optical thickness ofthe continuous, variable thickness, multi-cavity interference filtersused in the optical multiplexing devices disclosed here varies, ofcourse, with the physical thickness of the filter at various pointsalong the surface of the optical block. At each of the multiple ports ofthe optical block associated with an individual signal channel, theoptical thickness of the interference filter is tuned to transmit thedesired wavelength sub-range(s). It will be apparent to those skilled inthe art in view of this disclosure that the thickness and composition ofthe layers of the continuous filter can be selected to suit the spectralprofile required for any given application of the optical multiplexingdevice. It will be apparent also, that the continuous filter can becontinuously variable in its thickness, linearly or otherwise, ordiscontinuously variable in its thickness. In certain preferredembodiments, the thickness of the filter at each port is substantiallyconstant, increasing (or decreasing) in thickness only between one portand the next.

The continuous, variable thickness, multi-cavity interference filtersused in the optical multiplexing devices disclosed here have manyadvantages over prior known filtering devices. They can be produced tocoat the entire operative portion of a surface of the optical block in asingle coating step, tunable at each "bounce point" (e.g., byappropriate placement of associated lens apparatus, collimeters, etc.)to exact wavelengths of ±0.1 nm. When manufactured with durablematerials to form dense layers of near unity packing density, they arestable over time and with respect to humidity. A large number of opticalblocks can be coated simultaneously with the interference filters in asingle coating run, thereby substantially reducing the cost of theoptical multiplexing device. They are readily manufactured comprisingmultiple cavities, which are coherently coupled using a quarter wavethickness layer in accordance with known techniques. The effect of usingmultiple cavities, as described above, is to produce a filter with anincreased slope of the spectral skirts, along with a wider transmissionzone. As described above, both of these effects offer advantages overother types of filtering devices, such as etalons and diffractiongratings. Since the filters can be formed by deposition directly onto asurface of the optical block, no epoxy need be used in the mounting ofthe filter so as to be in the path traveled by the optical signals. Thestability of the filter is enhanced, since it is formed on the opticalblock, and need not be positioned and aligned in a separate mountingoperation. As noted above, the center wavelength for each of themultiple signal channels can be tuned by simply moving a GRIN lenscollimator or the like associated with each of the signal channels aslight measure in the direction of the varying thickness of thecontinuous filter. By so moving the associated lens apparatus, it isaligned with the desired signal wavelength. In this fashion theuncertainty of achieving the correct center wavelength in themanufacture of discreet filter elements is substantially overcome.

A dense channel wavelength division multiplexing device is illustratedin FIG. 1, employing a continuous, variable thickness, multi-cavityinterference filter to form an ultra-narrow bandpass filter at each ofeight separate ports on an optical block. This multiplexing device hasthe ability to multiplex individual, separate wavelength signals into acommon fiber optic carrier line and/or to demultiplex such signals. Forsimplicity of explanation, only the demultiplexing functionality isdescribed here in detail, since those skilled in the art will readilyunderstand the correlative multiplexing functionality. That is, thoseskilled in the art will recognize that the same device can be employedin reverse to multiplex optical signals from the individual channels.Typical specifications for an optical multiplexing device in accordancewith the preferred embodiment illustrated in FIG. 2 include thoseprovided in Table A.

                  TABLE A                                                         ______________________________________                                        Number of Channels 8                                                          Channel wavelength 1544-1560                                                  Channel spacing    2 nm ± 0.2 nm                                           Minimum Isolation  20 dB to 35 dB                                             Insertion loss (total)                                                                           less than 6 dB                                             Fiber type         single mode, 1 meter pigtail                               Operating temperature range                                                                      -20° C. to +50° C.                           ______________________________________                                    

The optical multiplexing device of FIG. 2 meeting the specifications ofTable A, is seen to include an optical block 10 which, preferably, is astable glass substrate. A means for projecting collimated light, such asa fiber optic GRIN lens collimator 12 or the like, couples highlycollimated light 14 to the optical block at a slight angle through ahole or facet in surface 16 of the optical block. In accordance with onepreferred embodiment, the optical block has a thickness "a" of 5 mm, anda length "b" of 14.1 mm or more, and a refractive index of about 1.5.The collimated light preferably has a divergence of not more than about0.15° and the tilt angle "c" at which the collimated light enters theoptical block is about 15°. Thus, multicolor or multi-wavelength lightcarried by an optical fiber (preferably a single mode fiber) carrier iscollimated by lens means 12 and directed through an optical port 18 insurface 16 of the optical block 10, from which it passes within theoptical block to the opposite surface 20. A graded wavelengthall-dielectric narrow bandpass filter 22 is carried on surface 20 of theoptical block. Specifically, filter 22 is a continuous, variablethickness, multi-cavity interference filter as described above, and,most preferably, is a continuous linearly variable filter. Lightentering the optical block at optical port 18 first strikes oppositesurface 20 at output port 24. Filter 22 is transparent at output port 24to a sub-range of the wavelengths included in the collimated light 14.Specifically, light 26 passes through port 24 of the optical blockpreferably to a collimating lens means 28 associated with a first signalchannel. The optical signal passed by port 24 is thereby transmitted tooptical fiber, preferably single mode fiber 30, as a demultiplexedsignal.

The continuous filter 22 at port 24 is reflective of wavelengths whichare not "in-band" of the filter at that location. This reflected light32 is reflected from surface 20 of the optical block back to surface 16.Surface 16 carries a broadband high reflector film or coating 34. Highreflector film 34 does not cover optical port 18, so as to avoidinterfering with the passage of collimated light 14 into the opticalblock at that location. The reflected light 32 from the first outputport 24 is reflected at surface 16 by reflector film 34 back to surface20 of the optical block. The collimated light 14 enters the opticalblock at optical port 18 at a tilt angle of about 15°, where it refractsaccording to Snell's Law to an angle of approximately 9.9° and thenbounces between the opposite parallel surfaces 16 and 20 of the opticalblock. Thus, light 32 is reflected by reflector film 34 to strikesurface 20 of the optical block at a second location 36 corresponding toa second output port of the optical block. At the location of outputport 36, the continuous, variable thickness, multi-cavity interferencefilter 22 is transparent to a different wavelength or sub-range ofwavelengths than it is at output port 24. For dense channel wavelengthdivision multiplexing applications, the wavelength separation betweeneach of the multiple ports linearly spaced along surface 20 of theoptical block is preferably about 2 nm or less. Thus, at outport port 36an optical signal corresponding to a second channel is transmittedthrough the filter 22 to a collimating lens 38 and from there to fiberoptic carrier 40. As at the first output port 24, the interferencefilter 22 at output port 36 reflects light which is not in-band at thatlocation. Thus, the remaining portion 42 of the collimated light 14which first entered the optical block at optical port 18 is reflectedback from port 36 to the high reflector 34 on opposite surface 16 of theoptical block, and from there it is reflected or bounced back to a thirdoutput port 44. In similar fashion, the reflected wavelengths thencontinue to cascade in a zigzag or "multiple-bounce" path down theoptical block, with the optical signal for each individual channel beingremoved by successive bounces at surface 20 of the optical block.

As seen in FIG. 2, therefore, the zigzag path of light travel throughoptical block 10 causes the reflected wavelengths to strike, in turn,the additional downstream output ports 46, 48, 50, 52 and 54. At each ofthese multiple ports, the demultiplexed optical signal is passed to anassociated collimating lens, each communicating with a correspondingsignal carrier line or other destination. While preferably the filter 22is reflective of all wavelengths which are not in-band at each of themultiple output ports, in certain applications it would necessarily bereflective only of the wavelengths of optical signals which had not beenextracted at upstream output ports, that is, at output ports encounteredpreviously in the multi-bounce cascade sequence. Also, those skilled inthe art will understand from this description that the opticalmultiplexing device of FIG. 2 is equally suitable for use in combiningthe optical signals of the eight individual channels. Thus, the multipleports in surface 20 would be input ports and optical port 18 would be anoutput port. The cascading would then proceed downstream from the bottom(as viewed in FIG. 2) of the optical block toward the top.

For an optical block of 5 mm thickness, as recited above for opticalblock 10 of FIG. 2, with a tilt angle of 15° leading to a bounce angleof 9.9° within the optical block, the linear spacing of the individualoutput ports (TAN 9.9!×2×5 mm) would be 1.76 mm. Thus, continuousinterference filter 22 on surface 20 of the optical block should be atleast 14.1 mm in length (8×1.76 mm). The total distance traveled by theoptical signal associated with the last of the eight channels (5 mm×8channels×2 bounces) would be 80 mm. The total beam spread (80 mm TAN-1SIN-1! SIN! 0.15/1.5!) would be about 0.138 mm. The total loss,therefore, for a 0.5 mm beam would be about 1.9 dB. Accordingly, it willbe appreciated by those skilled in the art that the optical multiplexingdevice illustrated in FIG. 2 as described above, is suitable todemultiplex numerous individual wavelength channels out of an incidentlightbeam in a very efficient manner due to the minimal beam divergenceincurred. The total beam spreading for the preferred embodimentdescribed above would be approximately 40% for a half millimeter beam,which produces the aforesaid loss of only 1.9 dB or less than 0.25 dBper channel cascaded through the device. More specifically, thoseskilled in the art will recognize that the multiple-bounce cascadingtechnique achieved with a continuous, variable thickness, multi-cavityinterference filter deposited directly on the surface of an opticalblock provides an optical multiplexing device having performancecharacteristics, including cost and simplicity of construction,reliability of performance, compactness, etc., which are significantlyimproved over prior known devices.

In the alternative preferred embodiment illustrated in FIG. 3,collimated light 60 from a lens arrangement 62 communicating with asingle mode optical fiber 64 passes into optical block 66 at opticalport 68 substantially in accordance with the embodiment of FIG. 2described above. Thus, the light passes through optical block 66 to theopposite, multi-port surface 70 of the optical block, striking it firstat output port 72. A continuous, variable thickness, multi-cavityinterference filter 74 extends over surface 70 to provide a narrowbandpass filter at each of the multiple output ports 72, 76, 78 and 80.As in the embodiment of FIG. 2, the filter 74 is transparent to adifferent wavelength at each such port, whereby the single opticalsignal associated with channels 1, 3, 5 and 7, respectively, aretransmitted to corresponding lens apparatus and fiber optic waveguides.On surface 82 of the optical block, a reflective film 84 is provided tocooperate with interference filter 74 on surface 70 to achieve themulti-bounce cascading within the optical block. In accordance with thispreferred embodiment, however, reflective film 84 also forms anarrowband filter at each bounce location. Thus, each bounce location atsurface 82 of the optical block is an additional output port at whichthe optical signal associated with an additional channel is passed to anassociated lens arrangement and fiber optic carrier line. Morespecifically, reflective film 84, which preferably is also a continuous,variable thickness, multi-cavity interference filter, and mostpreferably a continuously linearly variable interference filter, istransparent to the wavelength of the optical signal of channel 2 atoutput port 86 and reflective of the other wavelengths. Similarly, it istransparent to the optical signal of channel 4 at output port 88 and,again, reflective at that location to other wavelengths. Output port 90is transparent to the optical signal of channel 6 and, finally, outputport 92 is transparent to the optical signal of channel 8.

It will be recognized by those skilled in the art that the opticalmultiplexing device illustrated in FIG. 3 can provide highly efficientand compact multiplexing and demultiplexing functionality. Forcollimated light having a divergence of 0.15° and entering optical port68 at a tilt angle of about 12°, the optical block may advantageously beformed of fused silica and have a width of about 10.361 mm. Linearspacing of the output ports on each of surfaces 70 and 82 is preferablyabout 3.067 mm, yielding an overall linear dimension of approximately 15to 20 mm for the optical block. Generally, it is preferred in devices ofthe type discussed here, to have a low entry angle or tilt angle (wherezero degrees would be normal to the surface of the optical block) atwhich light passes through the optical port (measuring the angle of thelight outside the optical block). A low entry angle reduces polarizationdependent effects. It also reduces adverse effects of collimated lightdivergence on filter performance, since a lower entry angle results inmore closely spaced bounce points within the optical block and a shortertravel path for the light. Typically, the entry angle is less than 30°,being preferably from 4° to 15°, more preferably 60 to 10°, mostpreferably about 8°.

FIG. 4 illustrates another preferred embodiment, wherein the reflectivefilm on the second surface 82 of the optical block 66 comprises multipleseparate elements 120-126. The other features and elements are the sameas the corresponding features and elements of the embodiment of FIG. 3,and are given the same reference numbers. The individual reflective filmelements 120-126 can be deposited, e.g., by a sputtering process or thelike, directly onto the surface 82 of the optical block or onto separatecarrier substrates to be individually positioned and attached to theoptical block.

Epoxy or other adhesive may be used to attach the reflector elements.The individual reflector films can be broadband reflectors, operatingsubstantially as reflector film 34 in the embodiment of FIG. 1.Alternatively, they may operate as multiple additional ports, i.e., asbandpass filters or dichroic filters substantially in accordance withthe principles of reflective film 84 of the optical multiplexing deviceof FIG. 3.

Additional alternative embodiments will be apparent to those skilled inthe art in view of this disclosure, including, for example, opticalmultiplexing devices wherein two (or more) solid optical substrates arecoated, one or both (or all) with continuous, variable thicknessinterference filters to form multiple ports on a single mono-planersurface as illustrated and described above, and then joined together toform the optical block.

The film stack structure for the continuous, variable thickness,multi-cavity interference filter 22 in the preferred embodimentillustrated in FIG. 2 is illustrated in FIG. 5 and 6. Preferably, thethickness of each alternating layer (for example, of niobium pentoxideand silicon dioxide), as well as the total thickness of the film stack,is precisely controlled, most preferably within 0.01% or 0.2 nm overseveral square inches of area. In addition, the film stack should bedeposited with very low film absorption and scatter, and with a bulkdensity near unity to prevent water-induced filter shifting. Suchultra-narrow, multi-cavity bandpass filters have excellent performancecharacteristics including: temperature and environmental stability;narrow bandwidth; high transmittance of the desired optical signal andhigh reflectance of other wavelengths; steep edges, that is, highlyselective transmissivity (particularly in designs employing threecavities or more); and relatively low cost and simple construction. Asshown in FIG. 5, the filter is a three cavity filter, wherein onecavity, the "first cavity," is immediately adjacent the glass substrate.A second cavity immediately overlies the first cavity and the thirdcavity immediately overlies the second cavity and, typically, is exposedto the ambient atmosphere. In FIG. 6 the structure of the "first cavity"is further illustrated. A sequence of alternating layers of high and lowindex materials are deposited. Preferably, the first layer immediatelyadjacent the glass surface is a layer of high index material, followedby a layer of low index material, etc. Each of the high index layers 90is one quarter wave optical thickness (QWOT) or three quarter waves orother odd number of QWOTs. The low refractive index layers 92 which areinterleaved with the high refractive index layers 90 are similarly onequarter wave optical thickness or other odd number of QWOTs inthickness. Preferably there are about six sets of high and low layers,forming the bottom-most dielectric reflector 94. A one QWOT thick layer95 of high refractive index material separates dielectric reflector 94from cavity spacer 96. Cavity spacer 96 typically comprises 10 to 20alternating layers of high and low index materials, wherein each of thelayers is an even number of QWOTs in thickness. The second dielectricreflector 98 preferably is substantially identical to dielectricreflector 94 described above. The second and third cavities aredeposited, in turn, immediately upon the first cavity and preferably aresubstantially identical in form. The thickness of the interferencefilter layers varies along the length of the multi-port surface of theoptical block, as described above. Thus, the physical thickness of aQWOT will vary along the multi-port surface. Various alternativesuitable film stack structures are possible, and will be apparent tothose skilled in the art in view of this disclosure.

It will be apparent from the above discussion that various additions andmodifications can be made to the optical multiplexing devices describedhere in detail, without departing from the true scope and spirit of thisinvention. All such modifications and additions are intended to becovered by the following claims.

I claim:
 1. An optical multiplexing device comprising an optical blockhaving an optical port transparent to multiple wavelength collimatedlight, a continuous, variable thickness interference filter extending ona multiport surface of the optical block and forming multiple portsarrayed in spaced relation to each other along the multiport surface, atleast one of the multiple ports of the continuous, variable thicknessinterference filter being transparent to a different wavelengthsub-range of the multiple wavelength collimated light than at leastanother of the multiple ports, and means for cascading light along amulti-point travel path from one to another of the multiple ports. 2.The optical multiplexing device in accordance with claim 1 wherein theinterference filter is continuously variable.
 3. The opticalmultiplexing device in accordance with claim 1 wherein the interferencefilter is continuously linearly variable.
 4. The optical multiplexingdevice in accordance with claim 1 wherein the means for cascading lightcomprises a reflective coating on a second surface of the optical block.5. The optical multiplexing device in accordance with claim 4 whereinthe second surface of the optical block is spaced from and substantiallyparallel to the multiport surface.
 6. The optical multiplexing device inaccordance with claim 4 wherein the reflective coating is continuousover the second surface, being at least co-extensive with saidmulti-point travel path.
 7. The optical multiplexing device inaccordance with claim 6 wherein the reflective coating is a broadbandhigh reflector film coating which is substantially uniformly reflectiveof all of said sub-ranges of the multiple wavelength collimated light.8. The optical multiplexing device in accordance with claim 6 whereinthe reflective coating forms multiple additional ports arrayed in spacedrelation to each other along the second surface, the reflective coatingbeing transparent at each of the multiple additional ports to adifferent wavelength sub-range of the multiple wavelength collimatedlight, and reflective of other wavelengths thereof.
 9. The opticalmultiplexing device of claim 4 wherein the reflective coating comprisesmultiple discreet reflective film elements arrayed in spaced relation toeach other along said second surface.
 10. The optical multiplexingdevice in accordance with claim 4 wherein the means for cascading lightfurther comprises means for directing multiple wavelength collimatedlight into the optical block through the optical port at an angle to themultiport surface between 4° and 15°.
 11. The optical multiplexingdevice in accordance with claim 1 wherein each one of the multiple portshas an associated lens means for focusing collimated light passed bythat one of the multiple ports.
 12. The optical multiplexing device inaccordance with claim 11 wherein the lens means comprises a GRIN lenscommunicating with optic fiber.
 13. The optical multiplexing device inaccordance with claim 1 wherein the optical block comprises a solidblock of material substantially transparent to said multiple wavelengthcollimated light and selected from the group consisting of glass andfused silica, the continuous, variable thickness interference filterbeing on an outside surface thereof.
 14. The optical multiplexing devicein accordance with claim 1 wherein the optical block comprises anenclosed chamber.
 15. The optical multiplexing device in accordance withclaim 1 wherein the optical block is substantially rectilinear, with theoptical port being at a front surface of the optical block which isopposite and parallel the multiport surface of the optical block. 16.The optical multiplexing device in accordance with claim 15 wherein (a)the means for cascading light comprises on the front surface areflective film coating not extending over the optical port; (b) thereare at least eight of said multiple ports, each being a bandpass filtertransparent to a discreet wavelength sub-range separated from thewavelength sub-range of adjacent ones of the multiple ports byapproximately 2 nm; (c) collimated light passes through the optical portat an angle of approximately 6°-10° to the plane of the front surface,and (d) the multiple ports are linearly spaced from one another alongthe multiport surface.
 17. The optical multiplexing device in accordancewith claim 16 wherein the reflective film on the front surface of theoptical block is a broadband high reflector film coating.
 18. Theoptical multiplexing device in accordance with claim 15 wherein themeans for cascading light comprises on the front surface a reflectivefilm coating not extending over the optical port, the reflective filmcoating being a second continuous, variable thickness interferencefilter extending on said front surface of the optical block formingmultiple additional ports, the second interference filter beingtransparent at each of the multiple additional ports to a differentwavelength sub-range and reflective of other wavelengths of the multiplewavelength collimated light.
 19. The optical multiplexing device inaccordance with claim 18 wherein (a) there are at least four of saidmultiple ports and at least four of said multiple additional ports. 20.The optical multiplexing device in accordance with claim 1 wherein thecontinuous, variable thickness interference filter forms at each one ofthe multiple ports an all-dielectric narrow bandpass filter.
 21. Theoptical multiplexing device in accordance with claim 1 wherein thecontinuous, variable thickness interference filter is a multi-cavityinterference filter.
 22. The optical multiplexing device in accordancewith claim 21 wherein the continuous, variable thickness interferencefilter comprises a film stack forming at least three interferencecavities.
 23. The optical multiplexing device in accordance with claim 1wherein the continuous, variable thickness interference filter comprisesa film stack formed of alternating films of niobium pentoxide andsilicon dioxide.
 24. The optical multiplexing device in accordance withclaim 1 wherein the multiple wavelength collimated light consistssubstantially only of the combination of wavelength sub-ranges to whichcorresponding ones of the multiple ports of the continuous, variablethickness interference filter are transparent, whereby the continuous,variable thickness interference filter is transparent to substantiallyall of the multiple wavelength collimated light.
 25. The opticalmultiplexing device in accordance with claim 1 wherein each of themultiple ports is substantially reflective of all wavelengths of themultiple wavelength collimated light to which each other of the multipleports is substantially transparent.
 26. The optical multiplexing devicein accordance with claim 1 wherein the multi-point travel path from oneto another of the multiple ports has an entry angle at the optical portbetween 4° and 10°.
 27. The optical multiplexing device in accordancewith claim 26 wherein the entry angle is between 4° and 6°.
 28. Theoptical multiplexing device in accordance with claim 26 wherein theentry angle is about 4°.
 29. The optical multiplexing device inaccordance with claim 26 wherein the entry angle is about 6°.
 30. Theoptical multiplexing device in accordance with claim 1 having at least 8of said multiple ports formed by the continuous, variable thicknessinterference filter.
 31. The optical multiplexing device in accordancewith claim 1 having from 8 to 32 of said multiple ports formed by thecontinuous, variable thickness interference filter.
 32. The opticalmultiplexing device in accordance with claim 1 having from 8 to 16 ofsaid multiple ports formed by the continuous, variable thicknessinterference filter.
 33. The optical multiplexing device in accordancewith claim 1 wherein the continuous, variable thickness interferencefilter has a different optical thickness at each of the multiple ports,the optical thickness being substantially uniform over each respectiveone of the multiple ports.
 34. The optical multiplexing device inaccordance with claim 1 wherein at least one of the multiple portsformed by the continuous, variable thickness interference filter isdichroic.
 35. The optical multiplexing device in accordance with claim34 wherein the continuous, variable thickness interference filter atsaid at least one of the multiple ports is a long wavepass edge filter.36. The optical multiplexing device in accordance with claim 34 whereinthe continuous, variable thickness interference filter at said at leastone of the multiple ports is a short wavepass edge filter.
 37. Theoptical multiplexing device in accordance with claim 34 wherein each ofthe multiple ports is a dichroic filter formed by the continuous,variable thickness interference filter.
 38. The optical multiplexingdevice in accordance with claim 37 wherein the dichroic filter formed bythe continuous, variable thickness interference filter at each of themultiple ports has a transition point set at a boundary wavelength, theboundary wavelength at each of the multiple ports being separated 0.1 nmto 2.0 nm from the boundary wavelength of an adjacent one of themultiple ports.
 39. The optical multiplexing device in accordance withclaim 1 wherein the wavelength separation between each of the multipleports is 0.1 nm to 2.0 nm.
 40. The optical multiplexing device inaccordance with claim 1 wherein the wavelength separation between eachof the multiple ports is 0.1 nm to 1.0 nm.
 41. The optical multiplexingdevice in accordance with claim 1 wherein the wavelength separationbetween each of the multiple ports is about 1.0 nm.
 42. The opticalmultiplexing device in accordance with claim 1 wherein the wavelengthseparation between each of the multiple ports is about 0.1 nm.
 43. Theoptical multiplexing device in accordance with claim 1 furthercomprising a light detector optically coupled to a corresponding one ofthe multiple ports to receive light passed from the optical blockthrough said corresponding one of the multiple ports.
 44. The opticalmultiplexing device in accordance with claim 1 wherein each of themultiple ports formed by the continuous, variable thickness interferencefilter is optically coupled to a corresponding light detector of a lightdetector array.